Electro-osmosis relates to the fact that an electrical potential having a direct current causes cations to move from the positive end (anode) toward the negative end (cathode), thereby dragging with it water molecules by a viscous action.
One of the main drawbacks of the electro-osmosis techniques is the drop of potential that occurs at the anode. A drop of potential at the anode can be explained by the decrease of water content of the soil (or sludge) adjacent to the anode. It can also be explained by the decrease of the conductivity of the pore water, by the generated gases during the electrolysis, and by the dissolution of the material constituting the anode. Moreover, a drop of potential can also be explained because of an inappropriate contact between the anode and the soil or sludge. Thus, the voltage gradient which is directly applied to the soil or sludge can only be a fraction of the voltage applied to the electrodes.
The abstract of Japanese Patent No. 60,114,315 discloses a method of dehydrating an organic sludge. In this process, the organic sludge is impregnated with saline and dehydrated by using both compression and electro-osmosis. In particular, the sludge is charged into a cylinder comprising electrodes connected to compressing rods. Each of the electrodes comprises a filter cloth disposed between the sludge and the electrode. Moreover, in this process, the sludge is flocculated by adding flocculants.
U.S. Pat. No. 4,376,022 discloses an apparatus for concentrating an aqueous sludge by electro-osmosis. An electric field is applied to a sludge which is disposed between two electrodes. The sludge is thus treated by electro-osmosis and is continually circulating from top to bottom of the apparatus between the electrodes so as to expel water from the sludge. The electrodes comprise endless belts for moving the sludge, and filter bands disposed between the electrodes and the sludge.
U.S. Pat. No. 5,401,375 describes an electro-endomosis type dehydrator. In this apparatus, the sludge is submitted to a voltage applied between a rotary drum having electrodes on its cylindrical wall and a press belt serving as a cathode. The dehydrator also comprises a filtrating cloth disposed between the press belt and the rotary drum.
U.S. Pat. No. 5,891,342 describes a sludge dewatering process wherein the sludge is first flocculated and then applied to a first pre-dewatering belt wherein the water is drained away under gravity. Then, the sludge undergoes a second pre-dewatering step on another belt before entering into a press zone. In this zone, the sludge is transported between two belts and a force is applied to the sludge.
U.S. published Patent Application 2003/0150789 describes an apparatus for electro-dewatering and recovery of solid material from a solid liquid mixture. The apparatus comprises a rotary drum of cathodic nature and two conveyor belts which are driven around the rotary drum. Adjacent to the rotary drum, one of the conveyor belts moves progressively toward the other belt so as to compress the sludge. The other belt comprises anodic patches and the inner porous conveyor belt is thus disposed between the cathode and the anode.
Gingerich et al. in Water Environment Research, May/June 1999, pages 267-276, Volume 71, Number 3, describe a laboratory unit cell electro-osmosis device used for dewatering sludge. This device comprises a cylinder body including a piston and a bottom support plate, the latter acting as electrodes. The piston is activated by a compressed air inlet and a filter cloth is disposed on the bottom support plate, i.e. between the sludge and the bottom support plate. This document also describes various experimental series that have been made with this device, wherein different values of constant voltage or constant pressure have been tested.